1. Field of the Invention
The present invention relates to a protective system, in particular an emergency braking system for a machine tool, e.g., a circular saw, which develops its protective effect within a very short time period which is generally in the range of several milliseconds (typically 1 to 50 ms) in order to protect a user of the machine tool from injuries in hazardous situations.
2. Description of Related Art
At the present time there are essentially three different approaches for implementing such a protective system for table and format circular saws, which are intended to prevent a user from coming into contact with the revolving saw blade or incurring a serious cut injury.
The protective system of a U.S. manufacturer, provided and marketed under the company name SawStop Inc., is an emergency braking system which allows the machine tool to be braked as the result of a direct intervention in the saw blade by a braking actuator system as soon as an appropriately designed sensor has detected a hazardous situation. Using a hot-wire triggering device, a rotatably mounted aluminum block is pushed into the toothing of the running saw blade with the aid of a pretensioned spring, the aluminum block becoming wedged therein and thus absorbing the rotational energy of all geometries of the machine tool which are revolving during the sawing operation. As a side effect, this one-sided application of force on the cutting tool is used to lower the saw blade into the saw table, using a specially designed suspension situated on the saw table. With the aid of this system it is possible to avert serious bodily injuries to the operator of the machine tool who triggers the protective mechanism. A disadvantage is the direct action on the machining tool, i.e., on the cutting geometry of the saw blade, since additional hazard potential for the operator results when parts of the toothing break off. Furthermore, restoring the operability of the protective system requires replacement of the brake unit, including the saw blade, with a replacement unit which is ready for use, and which the operator must have in stock in order to allow continued operation after a braking operation has taken place using the protective function. This entails significant consequential costs and a corresponding time expenditure for the procurement and installation. In addition, it may be assumed that all components affected by the braking operation, i.e., all revolving geometries of the machine tool, are subjected to severe stresses during the deceleration phase. Neither the manufacturer nor applicable publications pertaining to this system provide data concerning the fatigue strength of the unit.
Another approach involves the use of a protective system exclusively for lowering the saw blade into the saw table without initiating a braking operation of the saw blade. With the aid of a pyrotechnic ignition charge, the saw blade including the main shaft and its bearing are removed from the hazard zone, thus allowing severe injury to the operator to be prevented. A disadvantage of this type of protective system is the necessity of moving relatively large masses, in the form of the systems to be lowered, under severe time constraints in the millisecond range. The pyrotechnic ignition devices necessary for this purpose, which have proven to be indispensable for this protective actuator system, also result in costly partial reversibility, which limits immediate resumption of operations of the machine tool including a protective system which is ready for use, and also imposes time-related and organizational constraints. In addition, due to space limitations and a very specific operating procedure, this protective system is suitable only for fairly large stationary equipment, for example circular table saws, which allow such a design in their interior. In contrast, this system is ruled out for use on smaller hand-operated devices such as compound miter saws and miter saws, for example.
A disclosure from a publicly funded project named “Cut-Stop” (VDI/VDE/IT) regarding a protective system for format circular saws from the Institut für Werkzeugmaschinen (IFW) [Institute for Machine Tools], University of Stuttgart, describes an approach which, using a special shape of a disk brake system, namely, a self-amplifying wedge brake, brings the main shaft of the machine tool, and thus the saw blade, to a standstill. With the aid of a pyrotechnic ignition device, a wedge is accelerated and subsequently pushed between a stationary wedge guide, in the form of a modified brake caliper, and the rotating brake disk. The system acts in a self-locking manner for the selection and combination of specific wedge angles α and brake lining values μ, so that, using this design, the particular time demands on the braking operation may be met as a function of the mass inertia to be decelerated. However, as described in the cited publication, the disadvantage of this protective system is that subsequent to the triggering of the protective actuator system it is necessary to replace the complete brake unit due to the wedge which gets jammed in the friction pairing. The time required for carrying out this operation is approximately 10-12 minutes for complete restoration of system operability. Thus, here as well, there is partial limitation of the immediate system reversibility.
A safety brake for elevators is known from published German patent document DE 195 36 995 A1, which has a unit which brakes and optionally stops the drive via speed-dependent deceleration when a predetermined maximum speed of the transport means is exceeded. The safety brake of published German patent document DE 195 36 995 A1 acts, as a function of the rotational speed, directly on the drive pulley of the cable-driven transport unit, and is able to limit the rotational speed thereof. The safety brake, which is designed as a centrifugal force brake, also has a device for boosting the braking force as a function of the conveying speed.